Certain radiation curable coatings and films such as those formed from the acrylates, particularly trimethylol propane triacrylate, trimethacrylate, pentaerythritol triacrylate, and hexanediol diacylate or methacrylate, are in great demand because of their rapid curing properties. However, these compounds are normally highly viscous liquids or solids and thus are unsuitable as diluents for other polymeric components of a radiation curable formulation. Indeed, such compounds themselves require the incorporation of a diluent or solvent for uniform substrate coating, control of coating thickness and curing at low temperatures. Accordingly, low viscosity monofunctional diluents are usually included in their formulations. While these diluents are reactive, they materially reduce the cross-linked density of the finished product and consequently lower abrasion resistance and ability to withstand chemical attack.
Although solvents have been used to reduce viscosity, they are detrimental in radiation curing due to their volatility which presents problems for uniform composition control unless their evaporation prior to radiant exposure is effected. Obviously, such procedure extends processing time and may pose environmental drawbacks.
To some extent, the drawbacks of high viscosity monomers can be reduced by curing at elevated temperatures. However, this alternative significantly adds to the cost of the overall operation in the expenditure of energy, temperature control and loss of more volatile components in the composition or blistering of the coating resulting from entrained volatiles.
Since acrylate monomers are not conducive to cationically induced radiation curing, they require free radical systems which are oxygen inhibited unless effected in an inert atmosphere, generally under a blanket of nitrogen. Although formulation with a photoinitiator which undergoes bimolecular reaction with a hydrogen donor minimizes the inhibitory effect of air, this benefit is realized at the expense of a greatly reduced cure rate. Also, it is found that polymerization or curing in free radical systems ceases almost immediately upon removal from the source of radiation; thus, the cured product often contains significant amounts of unpolymerized components. Accordingly, it is an aim of research to develop a multifunctional monomer having the beneficial properties of multifunctional acrylates but which is amenable to radiation curing at a rapid rate by cationically induced polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure.
A monomer having a functionality greater than 2 which is a low viscosity liquid and which can be polymerized cationically is greatly desired since such a monomer would allow greater control of the crosslink density of the cured product.
The inherent deficiencies of the acrylate systems can be partially overcome by the use of epoxy resins. Epoxy resins can be polymerized by normal radiation techniques using cationic photoinitiators such as iodonium, sulfonium and ferrocene salts of hexafluorophosphate, hexafluoroantimonate and hexafluoroarsonate to produce a tack free film. Although in such formulations tack free products are obtained, polymerization of the mixture is incomplete. It is well known that the polymerization of epoxy resins is extremely slow and requires as much as several days to achieve their ultimate physical properties. Thus, thermal post curing is often employed to increase the rate of or to complete the polymerization.
Certain allyl compounds also have been used as coatings; however these monomers and their oligomers are not readily curable by cationic radiation. Thermal curing is generally required to increase the rate of polymerization. While allyl ethers of polyethylene glycols are curable by UV light, they require a free radical initiated reaction which proceeds at a slow rate, generally over a period of from 2 to 10 hours in order to reach completion.
Finally, it is noted that the unsubstituted acrylates are sensitizers and skin irritants as well as being carcinogenic, so that specialized safety precautions must be taken to protect operators from exposure. Although alkoxylation has lessened irritancy of the acrylates, their carcinogenic properties are not reduced.
Accordingly it is an object of the present invention to overcome the above described deficiencies by an economical and commercially feasible composition and curing process.
Another object of this invention is to utilize a multifunctional cross-linking agent which is a liquid and which is more economically employed in an efficient cross-linking process.
Another object is to provide a non-toxic cross linkable homopolymeric compound suitable as a film or a substrate coating which possesses good adhesion, abrasion resistance and resistance to chemical attack.
Still another object is to provide a more economical process for cross-linking monomeric or polymeric vinyl or epoxy ethers which can be effected in the presence of air.
Another object is to provide a monomer which is curable at a rapid rate by cationically induced radiation.
These and other objects will become apparent from the following description and disclosure.